An Electrospray Ion Source will be interfaced to a Time-Of-Flight Mass Spectrometer incorporating a Pseudo-Random-Chopping technique. The TOF mass spectrometer will allow rapid scan rates and high mass analysis for multiple and singly charged ions delivered from the Electrospray Ion Source. Using a Pseudo-Random-Chopping approach for oscillating the ES ion beam across an orifice instead of the standard periodic chopping pattern can increase the duty cycle potentially by a factor of ten. This increased duty cycle increases sensitivity proportionally without sacrificing resolution. Assembly of the ES-TOF instrument and testing of the pseudo-random-chopping technique will be conducted under Phase I funding. This instrument will have strong commercial potential as a relatively inexpensive MS system which will allow very high mass analysis and on line liquid feed system detection with extremely good sensitivity. With ES as the interface this ES-TOF instrument can be used as LC/MS or a CZE/MS analyzer. More sensitive MS/MS applications could be achieved with this ES-TOF instrument in part due to the multiple charging possible in ES.